A transmission electron microscope (TEM) enables observers to form images of extremely small features, on the order of nanometers to fractions of Angstroms. TEM images allow analysis of the internal structure of a sample. In a TEM, a broad beam of electrons impacts the sample, and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site.
A thin TEM sample cut from a bulk sample material is known as a “lamella.” Lamellae are typically less than 100 nanometers (nm) thick, but for some applications a lamella must be considerably thinner. In the semiconductor industry, TEM and STEM analysis is becoming especially important to characterize the smallest and most critical structures. Lamella preparation is a critical step in TEM analysis. The continuing demand to reduce the size of transistors results in the need to further decrease the thickness of lamellae to provide samples that contain one discrete transistor structure only. The minimum feature size or “pitch” used in semiconductor manufacturing is moving toward 22 nm, so it will be desirable to produce lamella having a thickness of around 10 nm. Lamellae having thicknesses of less than 20 nm are challenging to produce in a reliable and repeatable manner.
Such thin lamellae are subject to mechanical failure due to the lack of structural integrity—warping, bending, and erosion of critical areas often occurs in very thin samples. Since the required thickness of lamellae is decreasing, there is a need for a method of providing and maintaining structural integrity of thin samples.
Methods for TEM lamella preparation typically use a focused ion beam (FIB) system. The accuracy of lamella thickness and the final lamella center location were based on the accuracy of the placement of FIB milling operations. In an automated work flow, milling is typically performed with respect to some feature or fiducial on the top surface of the substrate from which the TEM sample lamella is to be milled.
As lamellae less than 20 nm thick are desired, the required increased level of precision requires a higher operator skill. Further, the success rate of lamella samples decreases dramatically as the thickness decreases, for example, to less than 10 nm.
Prior art lamella reinforcing techniques, such as the method taught by Lechner in EP 2413126, involve shaping the lamella to leave certain areas thicker than others for structural support. Such methods leave “windows” of thinner regions surrounded by thicker regions, but such windows can limit the field of view which in turn affects the amount of information that can be obtained from the lamella. Windowing also adds complexity and process time. Further, a high level of operator skill is required to direct the focused ion beam to different regions to leave varying levels of thickness within the lamella. Thus, what is needed is an improved method and apparatus to reinforce lamella samples.